Higher-value or higher-grade encoding procedures are already known, for example amplitude shift keying (referred to as ASK), frequency shift keying (referred to as FSK) and phase shift keying (referred to as PSK). In most cases the procedure adopted is PSK which is least susceptible to interference or a combination of ASK and PSK in the form of quadrature amplitude modulation (referred to as QAM). The 4PSK-procedure is primarily used, because of the low level of expenditure involved. In order to obtain the four phase steps two alternating currents of the same frequency, phase-shifted through 90.degree., are respectively shifted through 180.degree.. Here too the expenditure is still very considerable.